Optical transceivers are used in optical communications systems to convert data from electronics format into optical format and vice versa. A typical optical transceiver comprises a number of separate optical, optical-electrical, and electrical components including a semiconductor laser (LD) or light emitting diode (LED), an LD or LED driver IC, a P-I-N photodiode or Avalanche Photodiode (APD), a trans-impedance amplifier (TIA) and a limiting amplifier (LA). An LD can be a Fabry-Perot (FP) laser, a distributed feedback (DFB) laser, or a vertical cavity surface emitting laser (VCSEL). An LED can be either an edge emitting type or a surface emitting type of LED. These are discrete devices typically packaged separately in device packages, such as a thin outline (TO) can, or integrated circuit packages, such as a j-lead surface mount package.
The semiconductor laser is usually packaged together with a power monitoring photodiode and coupled with a lens system to form a transmission optical sub-assembly (TOSA). The laser driver controls the laser operation and maintains a constant average laser output power through an auto-power control (APC) feedback loop, while modulating the laser to convert electronics data into optical data. The APC feedback loop, associated with the laser driver, compensates for laser power changes due to temperature variations and laser aging. The laser driver IC modulates the LD or LED output power to convert electronics data into optical data.
The P-I-N photodiode is usually packaged together with the TIA in a single package and coupled to a lens system to form a receiver optical subassembly (ROSA). The photodiode receives and converts optical data into electrical data in electric current format, while the TIA converts the input electric current into voltage with linear amplification. The limiting amplifier amplifies and converts the analog input voltage signal from the trans-impedance amplifier into a constant voltage output. The output of the limiting amplifier is usually in either PECL or CML (current mode logic) format. Other features, such as Loss of signal (LOS) detect and squelch function, are usually integrated into the limiting amplifier. LOS indicates if the photodiode is receiving data and squelch function completely shuts down the data output from LA when the input signal falls below a predetermined level. The laser driver and LA, along with APC loop and other passive circuit elements, are electrical components that are mounted onto a printed circuit board (PCB) called electrical subassembly (ESA).
The APC can be implemented with either analog or digital resistors to form a feedback loop by monitoring the laser output power through the monitoring photodiode mounted inside the laser package. Alternatively, a microprocessor can be used along with an A/D converter in connection with the laser monitoring photodiode and a D/A converter in connection with the laser driver to control the laser bias current, thus to set the laser average output power at constant. The same microprocessor can also be used to control the modulation amplitude of the laser driver, thus to control the laser operation extinction ratio.
Sometimes, it is desirable to monitor certain operational parameters of the optical transceivers, such as laser bias current, laser average output power, average power or optical modulation amplitude received by the receiver, voltage applied to the transceiver and transceiver temperature. This monitoring capability can be achieved through a digital monitoring controller that includes a few A/D converters, non-volatile memory, SRAM, memory mapping, lookup table, and digital communication interfaces, such as a serial interface. The same monitoring functionality can alternatively be accomplished through the use of micro-controllers along with some non-volatile memory. The monitoring circuitry and microprocessor or micro-controller being electrical components, are mounted with the laser driver and LA, along with APC loop and other passive circuit elements, are onto the same printed circuit board (PCB) to become part of the electrical subassembly (ESA).
The monitoring circuitry and microprocessor or micro-controller has increased the number of circuit elements mounted to the printed circuit board and increased optical transceiver costs and the requirement for PCB area. With an increased usage of PCB area to provide the increased functionality, it makes it more difficult to reduce the size of the optical transceiver. Additionally, the increased number of circuit elements increases power consumption for a given optical transceiver. Moreover, as optical transceivers contribute to overall system hardware costs, it is desirable to improve over the prior art.